J. Adv. Model. Earth Sys., 7, doi:10.1002/2014MS000397.

An empirical model relating U.S. monthly hail occurrence to large-scale meteorological environment.

John T. Allen
International Research Institute for Climate and Society, Columbia University, Palisades, NY

Michael K. Tippett
International Research Institute for Climate and Society, Columbia University, Palisades, NY

Adam H. Sobel
Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, and Lamont-Doherty Earth Observatory, Columbia University, New York, NY

Abstract

An empirical model relating monthly hail occurrence to the large-scale environment has been developed and tested for the United States (U.S.). Monthly hail occurrence for each 1x1 degree grid box is defined as the number of hail events that occur there during a month; a hail event consists of a 3 h period with at least one report of hail larger than 1 in. The model is derived using climatological annual cycle data only. Environmental variables are taken from the North American Regional Reanalysis (NARR; 1979-2012). The model includes four environmental variables convective precipitation, convective available potential energy, storm relative helicity, and mean surface to 90 hPa specific humidity. The model differs in its choice of variables and their relative weighting from existing severe weather indices. The model realistically matches the annual cycle of hail occurrence both regionally and for the contiguous U.S. (CONUS). The modeled spatial distribution is also consistent with the observed hail climatology. However, the westward shift of maximum hail frequency during the summer months is delayed in the model relative to observations, and the model has a lower frequency of hail just east of the Rocky Mountains compared to observations. Year-to-year variability provides an independent test of the model. On monthly and annual time scales, the model reproduces observed hail frequencies. Overall model trends are small compared to observed changes, suggesting that further analysis is necessary to differentiate between physical and nonphysical trends. The empirical hail model provides a new tool for exploration of connections between large-scale climate and severe weather.